Up until recently, almost all grinding rolls in pulverizing mills have been biased towards the grinding bowl by means of a mechanical coil spring. A few mills have used hydraulic pressure for biasing the rolls towards the grinding bowl, and although such hydraulic loading has many advantages, there has been one major drawback to the widespread adoption of such a hydraulic loading system. When using a mechanical spring for biasing the grinding rollers, there is an inherent spring rate characteristic built into it. Thus as the output requirements of the pulverizing mill are varied, resulting in a variation in the bed depth of the material between the grinding bowl and the grinding rollers, the force exerted by the spring, biasing the roller towards the bowl, is also changed. The spring is initially set with a given load, for example 50,000 pounds, when there is no material, such as coal, present on the bowl. Thus if the spring has a 50,000 pound per inch constant, it would initially be compressed one inch. When the bowl mill is operating at 100 percent capacity, there may be a two inch depth of coal on the bowl. This would result in a three inch compression of the spring, or a 150,000 pound force being exerted by the spring. If the mill is later run at 50 percent of maximum capacity, the depth of the coal bed on the bowl would be one inch, and the spring would then exert only a 100,000 pound force. This is desirable, because as the amount of pulverized material wanted from the mill increases, the pressure exerted by a mechanical spring also increases proportionally, thus permitting more material to be pulverized in a given time because of the increased force exerted by the spring.
This has not been the case with hydraulic loading systems in existance today. They are initially loaded to exert a predetermined force, for example 50,000 pounds, which force remains fairly constant regardless of the amount of material being pulverized. Thus if a mill is run at 50 percent capacity, the material is pulverized at a much faster rate than when it is run at 100 percent capacity. If the mill is feeding the material directly to a point of use, without an intermediate storage facility, this creates undue problems. Such is the case when a mill is used for supplying pulverized coal directly to a furnace of a steam generator, the output of which may vary from 30 to 100 percent at different times.